1. Field of the Invention
The present invention generally relates to a driving circuit, and more particularly, to a constant-current driving circuit.
2. Description of Related Art
FIG. 1 is a coupling diagram between a conventional constant-current driving circuit and a driven LED string. The conventional constant-current driving circuit 110 is for driving an LED string 121 formed by light emitting diodes (LEDs) 120 connected in series. The constant-current driving circuit 110 also receives an input signal VIN via an inductor 130 and performs voltage dividing on the output signal VOUT by using resistors 140 and 150 so as to obtain a voltage VR and conduct feedback control by using the voltage VR.
The constant-current driving circuit 110 includes a current source 111, a comparator 112, a timing control circuit 113, a P-type metal oxide semiconductor (PMOS transistor) 114 and an N-type metal oxide semiconductor (NMOS transistor) 115. The positive terminal of the current source 111 is coupled to the LED string 121 and the negative input terminal thereof is coupled to a common voltage level GND. The comparator 112 is employed for comparing the voltage VR with a fixed reference voltage FVREF to generate a comparison result CRS, then the timing control circuit 113 generates timing control signals TCS1 and TCS2 to respectively control the PMOS transistor 114 and the NMOS transistor 115 for turning on or off according to the comparison result CRS. By switching on the above-mentioned two MOS transistors, the required output signal VOUT is generated for driving the LED string 121.
In order to make the current source 111 work normally, the negative terminal voltage of the LED string 121 must be higher than or equal to the operation voltage VW of the current source 111 itself. That is, the output signal VOUT generated by the constant-current driving circuit 110 must have a voltage level greater than the sum of the total voltage drop across the LED string 121 and the operation voltage VW. For example, assuming a single LED 120 with a current I passes there-through has a forward bias voltage VF, the total voltage drop across N pieces of LEDs 120 should be N×VF. Thus, the voltage level of the output signal VOUT must be greater than N×VF+VW to ensure the normal operation of the current source 111.
The output signal VOUT of the constant-current driving circuit 110 is given by the following formula (1):VOUT=FVREF×((R1+R2)/R2)   (1)wherein R1 and R2 respectively represent the resistance of the resistors 140 and 150. Although the voltage level of the output signal VOUT is required to be sufficiently high to ensure the normal operation of the current source 111, however, in case where a variation of the forward bias voltage VF caused by process reasons occurs or the current I passing through the LED string 121 is adjustable, the total voltage drop across the LED string 121 may be less than the initially required total voltage drop. In addition, after the fixed reference voltage FVREF and the resistance of the resistors 140 and 150 are known, the output signal VOUT fails to be optimized to fit different LED strings 121, which makes a spare voltage drop applied to the current source 111 causing an efficiency loss and resulting in unwanted heat generation in the constant-current driving circuit 110.